When superabsorbent technology was first developed, only a high swelling capacity on contact of the superabsorbent polymer with liquids, referred to as the free swelling capacity in accordance with the free water absorption test (FWA), was the primary consideration. However, it was later realized that the water-absorbing polymers when present in a sanitary article, such as a diaper or incontinence garment, are subjected to mechanical load caused by movements of the person wearing the article. Thus, a new consideration arose in that the superabsorbent polymer, in addition to having a high swelling capacity, should also have a high capability for retaining liquid in accordance with the centrifuge retention capacity test (CRC) and a high absorbency under pressure in accordance with the absorbency under load test (AUL). A good discussion of the test for AUL can be seen in published European Patent Application No. 0 339 461 A1 (published Nov. 2, 1989; priority to U.S. Ser. No. 184,302 (Parent) and Ser. No. 334,260 (Continuation-in-Part), which Continuation-in-Part has issued as U.S. Pat. No. 5,147,343) to Kellenberger, assignor to Kimberly-Clark Corporation.
Published European Patent Application No. 0 437 816 A1 (published Jul. 24, 1991; priority to U.S. Ser. No. 464,798) to Kim and Nielsen, assignors to Hoechst Celanese Corporation, shows the wet-laid process for the manufacture of webs of superabsorbent polymer and cellulosic fiber. These webs are employed as core composites in disposable sanitary articles, such as those mentioned above. More particularly, disclosed is a process that involves blending superabsorbent polymer particulates with a liquid to form a slurry, followed by mixing cellulosic fibers with the slurry and then filtering to remove part of the liquid, and finally drying the resultant. The wet-laid process is also described in U.S. Pat. No. 4,605,401 (issued Aug. 12, 1986) to Chmelir and Künschner, assignors to Chemische Fabrik Stockhausen GmbH.
The journal article, “Keeping Dry with Superabsorbent Polymers”, Chemtech, (September 1994) by Buchholz, contains an excellent discussion of the conventional methods for making superabsorbent polymers, certain of which have sulfonate functional groups and certain of which have carboxylic acid functional groups. Also, Buchholz discussed various uses for superabsorbent polymers, such as in the above-noted sanitary articles, as well as in a sealing composite between concrete blocks that make up the wall of underwater tunnels and in tapes for water blocking in fiber optic cables and power transmission cables.
A good discussion of the methods for making superabsorbent polymers can also be seen in U.S. Pat. No. 5,409,771 (issued Apr. 25, 1995) to Dahmen and Mertens, assignors to Chemische Fabrik Stockhausen GmbH. More specifically, this patent mentions that commercially available superabsorbent polymers are generally cross-linked polyacrylic acids or cross-linked starch-acrylic-acid-graft-polymers, the carboxyl groups of which are partially neutralized with sodium hydroxide or caustic potash. Also mentioned is that the superabsorbent polymers are made by two methods, one being the solvent polymerization method and the other being the inverse suspension or emulsion polymerization method.
In the solvent polymerization method, an aqueous solution of partially neutralized acrylic acid for instance and a multi-functional network cross-linking agent is converted to a gel by radical polymerization. The resultant is dried, ground, and screened to the desired particulate size.
On the other hand, in the inverse suspension or emulsion polymerization method, an aqueous solution of partially neutralized acrylic acid for instance is dispersed in a hydrophobic organic solvent by employing colloids or emulsifiers. Then, the polymerization is started by radical initiators. Water is azeotropically removed from the reaction mixture after completion of the polymerization, followed by filtering and drying the resultant product. Network cross-linking typically is accomplished by dissolving a polyfunctional cross-linking agent in the monomer solution.
Furthermore, U.S. Pat. No. 5,154,713 (issued Oct. 13, 1992) to Lind and U.S. Pat. No. 5,399,591 (issued Mar. 21, 1995) to Smith and Lind, both of which patents are assigned to Nalco Chemical Company, describe new processes for making superabsorbent polymers, as a result of which the superabsorbent polymers display an increased, faster water absorption. The superabsorbent polymers are depicted as useful as absorbents for water and/or for aqueous body fluids when the polymers are incorporated into absorbent structures, such as disposable diapers, adult incontinence garments, and sanitary napkins.
General background with respect to various superabsorbent polymers and methods of manufacturing them can be seen in U.S. Pat. No. 5,229,466 (issued Jul. 20, 1993) to Brehm and Mertens; U.S. Pat. No. 5,408,019 (issued Apr. 18, 1995) to Mertens, Dahmen, and Brehm; and U.S. Pat. No. 5,610,220 (issued Mar. 11, 1997) to Klimmek and Brehm, all of which patents are assigned to Chemische Fabrik Stockhausen GmbH.
The disclosures of all above-mentioned patents and published patent applications are incorporated herein by reference.